1. Field of the Invention
The invention relates to dehydrating natural gas. More specifically, the invention provides an apparatus and process for dehydrating natural gas vehicle fuel while filling a compressed natural gas storage vessel for subsequent transfer to vehicle fuel tanks.
2. Description of the Related Art
In recent years, there has been considerable interest in using compressed natural gas as a fuel for vehicles. Applicant is named as inventor of several U.S. patents in this area, including: U.S. Pat. No. 5,370,159 entitled APPARATUS AND PROCESS FOR FAST FILLING WITH NATURAL GAS, and U.S. Pat. No. 5,385,176, entitled NATURAL GAS DISPENSING.
While growing concern about the environment, and more specifically about the quality of air, is spurring an interest in the use of compressed natural gas as a vehicular fuel, this technology presents several challenges.
These challenges include a significant corrosion problem. Corrosion fatigue is caused by a combination of corrosive agents found in natural gas--hydrogen sulfide, carbon dioxide, water (or water vapor)--which, together with the pressure cycling associated with the use of compressed natural gas equipment, results in the development of stress and corrosion cracks in the equipment. In order to limit these corrosive effects, standards have been developed for upper limits of water content in the compressed natural gas for use as a vehicular fuel.
In addition to the corrosion problem, water in compressed natural gas could freeze or form hydrates during handling, especially during desired fast-fill operations. The quantity of water in saturated natural gas at various pressures and temperatures can be estimated from correlations in the literature. Some of these correlations also show a hydrate-formation line indicating that solid hydrates form when the pressure of natural gas of a specific moisture content is suddenly reduced. For instance, if gas of typical pipeline composition (0.6 gravity) at 2000 psig and 120.degree. F. is expanded to 400 psig, hydrates form if the gas contains more than 15 pounds of water per million standard cubic feet ("MMSCF"). At pressures below about 150 psia, on the other hand, cooling to 32.degree. F. is necessary to precipitate a solid phase, when ordinary ice forms. The hydrates form more readily (i.e., at a higher temperature or lower pressure) with gases of greater density and less readily with very light gases. Thus, for example, at a pressure of 1,000 psia, hydrates form at about 62.degree. F. in natural gas of about 0.60 specific gravity, while they form at about 67.degree. and 71.degree. F., respectively, in gases of 0.75 and 1.00 specific gravity. Thus, it may be expected that compressed natural gas at pressures ranging from about 3,000 to about 5,000 psig, would be highly susceptible to the formation of hydrates if the gas is saturated with water vapor or contains a significant amount of water vapor. Cooling of compressed natural gas owing to pressure reduction necessarily takes place during natural gas vehicle fueling operations, particularly in the case of quick-fill systems.
It is known to remove water from gases by contacting the wet gas with a dehydrating solution that contains a substance that either absorbs or reacts with water. In this process, water vapor is transferred from the gas to the dehydrating solution and dried gas is obtained. Desirably, the dehydrating solution, now containing removed water as a liquid, is regenerated (i.e. dried) and recycled. The regeneration steps typically used include several stages of heating the dehydrating solution to drive off water as water vapor. The substantially water-free dehydrating solution is then recycled for contacting with wet gas.
Typically, the gas industry uses fairly complex gas dryers having very large capacities, ranging up to 75 MMSCF per day. These gas dryers include a wet gas dehydrating solution contactor and a dehydrating solution regenerator. U.S. Pat. No. 3,105,748 shows a glycol regenerator utilizing a still column for distilling wet glycol and removing water vapor. Glycol circulation rates vary from about 2 to about 5 gallons of glycol per pound of water to be removed. Thus, to remove 20 lbs. of water per MMSCF, from about 40 to about 100 gallons of glycol should be available for each MMSCF throughput.
It is also known to remove water from natural gas using dry-bed dehydration units. These units used a solid material called a desiccant to adsorb water from the natural gas. Typical desiccants included silica gel, activated alumina, and molecular sieves. When the desiccant became saturated, it had to be regenerated to restore its adsorptive capacity. Regeneration of a desiccant was usually accomplished by heating. Hot gas vaporized the water from the desiccant. A dry-bed dehydrator generally required at least two vessels filled with desiccant so one bed could be drying while the second was regenerating.
When dry-bed dehydration units were used to dry natural gas for use in natural gas vehicle filling stations, the drying process occurred simultaneously with filling of the compressed gas storage vessel. The gas was dried by taking a stream off of a lower stage of the compressor, delivering the gas to a drying system, and returning the dried gas to the next compressor stage. This process delivered compressed natural gas to the storage vessel which was hot due to the heat of compression. As the gas in the vessel cooled, it approached its dew point and there is a risk that residual contained water would condense.